The preparation of block copolymers is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive toward monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear block copolymer. Alternatively the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living diblock copolymer regions results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. Nos. 3,595,942 and Re. 27,145. In some cases what was desired was a random copolymer, such as an SBR, rather than a block copolymer. Random styrene butadiene copolymers or SBR are disclosed in U.S. Pat. Nos. 2,975,160, 4,547,560, 4,367,325 and 5,336,737.
Inventors desiring a low melt viscosity in block copolymers considered the use of random styrene and butadiene blocks, as disclosed in U.S. Pat. No. 3,700,633. One means of introducing transparency to block copolymers was to also provide for random blocks, such as in U.S. Pat. Nos. 4,089,913, 4,122,134 and 4,267,284.
When preparing random blocks of styrene and butadiene, so-called “tapered” blocks would result due to the fact that butadiene copolymerizes at a faster rate than does styrene. See, e.g. U.S. Pat. Nos. 5,191,024, 5,306,779 and 5,346,964. So in U.S. Pat. No. 4,603,155 the patentee prepared a block comprising multiple tapered blocks to achieve a more random copolymer. But in many cases the patentee relies on the continuous addition of both monomers or the use of randomizing agents to achieve a more random structure. Such techniques are disclosed in U.S. Pat. Nos. 3,700,633 and 4,412,087 and German patent applications DE 4420952, DE 19615533, DE 19621688, DE 195003944, DE 19523585, and DE 19638254. However, some randomizing agents will poison hydrogenation catalysts, and make the subsequent hydrogenation of the polymers difficult or impossible, so such randomizing agents must be avoided. Randomization agents containing N atoms are particularly prone to this problem.
While some improvements in properties have been made, it would be significant if it were possible to increase the stretching stiffness of a styrene/diene block copolymer without increasing the plasticity. What is also desired is a polymer having an increased polarity, while also having significantly lower melt and solution viscosity. Applicants have found that these improvements can be achieved by designing a polymer having a different structure in the diene block, wherein undesirable blockiness is avoided and undesirable effects occurring during post-polymerization hydrogenation treatments are also reduced or avoided.